feat: isNewerMajor + store write-refusal backstop (F5 SLICE-2, INV-16, #14)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/store.test.ts

@@ -107,3 +107,20 @@ describe("CoauthorStore", () => {
     expect(store.consumeSelfWrite(p)).toBe(false);
   });
 });
+
+describe("newer-major write refusal (F5 SLICE-2, INV-16 backstop)", () => {
+  it("update REFUSES to write a sidecar from a newer major, leaving bytes untouched", () => {
+    const store = new CoauthorStore(root);
+    const p = store.sidecarPath("docs/v2.md");
+    fs.mkdirSync(path.dirname(p), { recursive: true });
+    const v2 = { ...emptyArtifact("docs/v2.md"), schemaVersion: 2 };
+    fs.writeFileSync(p, serializeArtifact(v2), "utf8");
+    const before = fs.readFileSync(p, "utf8");
+    expect(() =>
+      store.update("docs/v2.md", (a) => {
+        a.threads = [];
+      }),
+    ).toThrow(/INV-16/);
+    expect(fs.readFileSync(p, "utf8")).toBe(before);
+  });
+});